1 Technical Field
The present invention relates to a battery characteristics learning apparatus that learns circuit constants of an equivalent circuit of a rechargeable battery (or secondary battery).
2 Description of Related Art
There has been known a method of calculating the internal resistance of a rechargeable battery and diagnosing the operating life (or service life) of the rechargeable battery based on the calculated internal resistance. Moreover, there also has been known a method of calculating the OCV (Open Circuit Voltage) of a rechargeable battery based on the internal resistance of the rechargeable battery, the terminal voltage of the rechargeable battery (or the voltage across the two terminals of the rechargeable battery) and the current flowing through the rechargeable battery and estimating the SOC (State of Charge) of the rechargeable battery based on the calculated OCV.
An equivalent circuit of a rechargeable battery is comprised of a perfect voltage source and circuit constants (i.e., ohmic resistance components and capacitance components) representing the internal resistance of the rechargeable battery. To calculate the internal resistance of the rechargeable battery in a transient response state where the current flowing through the rechargeable battery changes, it is necessary to first calculate the circuit constants of the equivalent circuit of the rechargeable battery. Further, there has been known, for example from Japanese Patent Application Publication No. JP2011047820A, a method of sensing both the terminal voltage of the rechargeable battery and the current flowing through the rechargeable battery for a predetermined time period and calculating the circuit constants based on values of the sensed terminal voltage and current.
Specifically, according to the method disclosed in the above patent document, the present values of the circuit constants R1 and C1 are calculated based on a regression line; the regression line is determined, on a two-dimensional coordinate plane (C1R1, R1 ) whose center point is represented by the previous values of C1R1 and R1, using a plurality of points in proximity to the center point.
Moreover, the circuit constants of an equivalent circuit of a rechargeable battery define a plurality of time constants that are different from each other. However, according to the method disclosed in the above patent document, the circuit constants are calculated based on the values of the sensed terminal voltage and current which are acquired without considering the time constants defined by the circuit constants. In other words, the time constants are not taken into account in the calculation of the circuit constants. Consequently, when the values of the sensed terminal voltage and current are acquired immediately after a change in the current flowing through the rechargeable battery, those circuit constants which define a large time constant cannot be accurately calculated whereas those circuit constants which define a small time constant can be accurately calculated. In contrast, when the values of the sensed terminal voltage and current are acquired after a long time from a change in the current flowing through the rechargeable battery, those circuit constants which define a small time constant cannot be accurately calculated whereas those circuit constants which define a large time constant can be accurately calculated. That is, in either case, it is impossible to accurately calculate all the circuit constants of the equivalent circuit of the rechargeable battery.